1. Field of the Invention
The present invention relates to a control channel transmitting method, a base station and a terminal, in particular, to a control channel transmitting method, a base station and a terminal for adaptively controlling a communication parameter with the use of a control channel.
2. Description of the Related Art
For example, in a communication system in the third generation, adaptive radio link control such as adaptive modulation/demodulation, HARQ (Hybrid Automatic Repeat request), scheduling or such is used, for the purpose of improving data packet transmission efficiency. The adaptive radio link control is carried out with the use of a separate or a shared control channel, and, a link parameter which is used in a data channel transmitted approximately simultaneously with the control channel, is notified of to each user terminal.
For example, in a case of the adaptive modulation/demodulation, the control channel transmits information such as a data channel modulating method, an encoding rate and so forth. In a case of the HARQ, the control channel transmits information such as packet numbers of packets transmitted via a data channel, the number of times of re-transmission and so forth. In a case of the scheduling, the control channel transmits information such as a user ID.
In HSPDA (High Speed Downlink Packet Access) standardized by 3GPP (Third Generation Partnership Project), the shared control channel called HS-SCCH (Shared Control Channel for HS-DSCH) is used, and control information such as that shown in Table 1, is transmitted, as disclosed by a non-patent document 1.
TABLE 1HS-SCCH physical channelChannelization-code-set information7 bitsModulation scheme information1 bit Transport-block size information6 bitsHybrid-ARQ process information3 bitsRedundancy and constellation version3 bitsNew data indicator1 bit Ue identity16 bits 
In a control channel transmitting method in the prior art used in the adaptive radio link control, in order to positively demodulate the control channel transmitting information required for data channel demodulation on a reception side, a fixed control channel format is used. Further, in order to avoid degradation in data channel transmission efficiency, an information amount is reduced to be bare minimum in the control channel transmitting method in the prior art, the control channel format to be used is fixed, and thus, a variable control channel format is not used.
As disclosed in a non-patent document 2, HSDPA control information shown in Table 1 undergoes rate matching to be totally 120 bits, after undergoing CRC attaching and convolution encoding of R=⅓. Then, QPSK modulation and spreading of SF=128 are carried out, and mapping is carried out in a physical channel of HS-SCCH.    Non-patent document 1: 3GPP TS 25.212 v5.9.0 (2004-06)    Non-patent document 2: 3GPP TS 25.211 v5.6.0 (2004-09)
In a next generation communications system, in order to achieve high-speed data transmission, MIMO (Multiple Input Multiple Output) transmission with the use of multi-carrier transmission or a plurality of antennas is used. In this case, it is possible to further improve transmission characteristics by carrying out adaptive control of a radio parameter for each sub-carrier or for each transmission antenna.
However, MIMO transmission has such a problem that the number of variable parameters increases, and the number of bits required for the control channel increases. Further, also for a case where the number of simultaneously multiplexing users via one frame increases, required information of the control channel increases in proportion to the number of users.
On one hand, the upper limit of a channel capacity simultaneously transmittable is defined by the number of channels which can be multiplexed orthogonally. Accordingly, when the information amount of the control channel increases, the number of data channels available for data transmission decreases, and as a result, the data transmission efficiency degrades.
Description will now be made with a case of user multiplexing by means of CDMA (Code Division Multiple Access) for example. FIG. 1 illustrates the number of codes available for data channels when the number of simultaneously multiplexing users is 1. FIG. 1 shows an example in which the number of simultaneously multiplexing users is 1, and, code multiplexing transmission of spreading of SF=8 is carried out for each of both the data channels and the control channels. When the number of codes required for transmitting the control channels is 1, the number of codes available for the data channels is 7 as all the remaining code resources which can be orthogonalized are used for the data channels.
FIG. 2 illustrates the number of codes available for data channels when the number of simultaneously multiplexing users is 4. When the number of simultaneously multiplexing users is 4 as in FIG. 2, 4 codes are used for the control channel, and thus, the number of codes available for the data channels is limited to 4. Accordingly, the data channel transmission efficiency per frame degrades in 4/7.
When the information amount of the control channel is reduced to be bare minimum in order to avoid degradation in the data channel transmission efficiency, a benefit of the adaptive radio link control may not be obtained, and thus, the transmission characteristics of the entire system may not be improved.
For example, in wide band multi-carrier transmission, communication quality differs for each sub-carrier due to frequency selective fading, and thus, it is possible to improve total throughput by carrying out communications with always selecting a sub-carrier having good communication quality.
Further, the same in MIMO transmission, when independent fading for each antenna branch occurs, it is possible to improve total throughput by always selecting a transmission antenna having good communication quality, or carrying out adaptive modulation/demodulation for each antenna. Thus, in the control channel transmitting method used for adaptive radio link control, total throughput should be improved without degradation in data channel transmission efficiency.